1. Technical Field
The present invention relates to a navigation apparatus that calculates a current position using the positioning information supplied from a GPS receiver.
2. Background Information
In general, a navigation apparatus has a GPS receiver connected thereto, and the position of a vehicle in which the navigation apparatus is installed is calculated on the basis of the positioning information supplied from the GPS receiver. The positioning information supplied from the GPS receiver includes an error depending on the actual location of GPS satellites. A method for calculating the error range using a decreasing rate of geometric accuracy called PDOP or HDOP is known (see, for example, Japanese Unexamined Patent Application Publication No. 6-148307). For example, combining the positioning using a GPS receiver and the positioning by an autonomous navigation sensor including a gyro and a velocity sensor can improve the detection accuracy of the vehicle position. For example, when the positioning result by an autonomous navigation sensor is within the error range, the positioning result by the autonomous navigation sensor is used. When the positioning result by the autonomous navigation sensor is out of the error range, the positioning result by the autonomous navigation sensor is not used and the positioning result using a GPS receiver is used.
However, since the calculation of the error range discussed in Japanese Unexamined Patent Application Publication No. 6-148307 uses the PDOP or the HDOP, it has a problem of low accuracy. The PDOP and the HDOP are calculated on the basis of the spatial distribution of the GPS satellites used for positioning. In addition, the elevation angles of the GPS satellites and the degree of bias of location of the GPS satellites are not reflected in the calculation of the error range using the value of the PDOP or the HDOP. Therefore, the error range varies widely. The case where GPS satellites having small elevation angles are distributed across a predetermined expanse, and the case where GPS satellites having large elevation angles are distributed across the same expanse in the vicinity of the zenith, are the same in the values of the PDOP and the HDOP, but differ widely in the error included in the positioning result and therefore differ also in the error range calculated therefrom.